[WebGPU-XR][Phase 4] Feature parity (composition/quad layers + binding-based features)

[RaananW]

Part of #18635. Depends on Phase 2 (and Phase 3 for correctness).

Phase 4 — Feature parity

Bring the remaining WebXR features to the WebGPU path, or degrade them gracefully.

Scope

• Composition / quad / cylinder / equirect / cube layers via XRGPUBinding.getSubImage (mirror the existing WebXRCompositionLayer path).
• Port features that currently build XRWebGLBinding + wrap WebGL textures — WebXRDepthSensing, WebXRRawCameraAccess, WebXRLightEstimation, WebXRSpaceWarp — through the Phase 0 binding abstraction.
• Where no WebGPU equivalent exists yet, degrade gracefully: disable the feature and emit a Logger.Warn rather than throwing.
• Audit every remaining feature for _gl / ThinEngine casts.

Acceptance

• All shipped WebXR features either work on WebGPU or disable themselves with a clear warning; WebGL2 behavior unchanged.